Detecting Cross-incompatibility of Three North American Apricot Cultivars and Establishing the First Incompatibility Group in Apricot

Abstract

Laboratory and orchard tests have shown that the apricot (Prunus armeniaca L.) cultivars 'Hargrand', 'Goldrich', and 'Lambertin-1' are cross- incompatible. All three cultivars are from North American breeding programs and have 'Perfection' as a common ancestor. In orchard tests, compatible pollinations resulted in 19% to 74% fruit set, while incompatible pollinations resulted in <2% fruit set. Microscopic examination showed that, in incompatible pollinations, pollen tube growth was arrested in the style, most frequently in its third quarter, and that the ovary was never reached. It is proposed that self-incompatibility in apricot is of the gametophytic type, controlled by one S-locus with multiple alleles, and that these three cultivars are S1S2.

Full text

1002 J. AMER. SOC. HORT. SCI. 121(6):1002–1005. 1996.
J. AMER. SOC. HORT. SCI. 121(6):1002–1005. 1996.
Detecting Cross-incompatibility of Three North
American Apricot Cultivars and Establishing the
First Incompatibility Group in Apricot
J. Egea and L. Burgos
Departamento de Mejora y Patología Vegetal, CEBAS–CSIC, Apd Correos 4195, 30.080–Murcia, Spain
Additional index words. Prunus armeniaca, fruit set, pollen tube growth, self-incompatibility
Abstract. Laboratory and orchard tests have shown that the apricot (Prunus armeniaca L.) cultivars ‘Hargrand’,
‘Goldrich’, and ‘Lambertin-1’ are cross-incompatible. All three cultivars are from North American breeding programs
and have ‘Perfection’ as a common ancestor. In orchard tests, compatible pollinations resulted in 19% to 74% fruit set,
while incompatible pollinations resulted in <2% fruit set. Microscopic examination showed that, in incompatible
pollinations, pollen tube growth was arrested in the style, most frequently in its third quarter, and that the ovary was never
reached. It is proposed that self-incompatibility in apricot is of the gametophytic type, controlled by one S-locus with
multiple alleles, and that these three cultivars are S1S2.
probable kinship. Burgos et al. (1993) did not find cross-incompat-
ibility after crossing other self-incompatible cultivars from the
same Spanish population. Many Spanish apricot cultivars seem to
have originated from crosses between north African cultivars
(mainly self-incompatible) and European cultivars (mainly self-
compatible) (Egea et al., 1988). Hence, a great degree of heterozy-
gosity is to be expected and self-incompatibility might be ex-
pected.
In the present work, cross-compatibility between some North
American cultivars was studied by examining pollen tube growth
using fluorescence microscopy and fruit set from controlled polli-
nations in the orchard.
Materials and Methods
Plant material. Studies were conducted in the apricot collection
at the Departamento de Mejora y Patología Vegetal, CEBAS–
CSIC, Murcia, Spain. Five-year-old trees of ‘Lambertin-1’,
‘Hargrand’, ‘Harcot’, and ‘Goldrich’ were used in this study.
These cultivars were obtained from breeding programs in the
United States and Canada. Five-year-old trees of the self-incom-
patible ‘Moniquí’ (Burgos et al., 1993) and the self-compatible
‘Canino’ and ‘Bebeco’ (Audergon et al., 1988) were also used to
check experimental methodology.
The parentage of the cultivars used in this study is shown in Fig.
1. ‘Goldrich’ is a seedling from ‘Sun Glo’ x ‘Perfection’ (Toyama,
1971). ‘Sun Glo’ was developed by O.H. Heider as a seedling of
unknown parentage, which originated in Washington in 1942
(Brooks and Olmo, 1972a). ‘Perfection’ was introduced in 1937 in
Waterville, Wash. (Brooks and Olmo, 1972a), and has been used
widely in American breeding programs despite its self-incompat-
ibility. ‘Geneva’ and ‘Naramata’ are seedlings of unknown parent-
age. ‘Geneva’ originated in Italy and was planted, evaluated, and
selected in New York by R. Wellington in 1934 (Brooks and Olmo,
1972a). ‘Lambertin-1’ was initially selected by the U.S. Dept. of
Agriculture–Agricultural Research Service at Fresno, Calif., and
evaluated under the designation P32-18.
Pollen collection and controlled pollinations. Flowers in the
balloon stage were collected from all cultivars to be used as male
parents. Anthers were removed from the flowers and placed in a
petri dish. The anthers were desiccated in a petri dish in a calcium
chloride desiccator for 24 h. then the pollen was stored in small
bottles and kept at 4 °C.
Received for publication 29 Jan. 1996. Accepted for publication 24 Apr. 1996. We
acknowledge to Adela Martinez Adsuar for technical assistance. This paper has
been supported by U.E. project 8001-CT 90-016 (PL 90 0022). The cost of
publishing this paper was defrayed in part by the payment of page charges. Under
postal regulations, this paper therefore must be hereby marked advertisement solely
to indicate this fact.
European apricot (Prunus armeniaca) cultivars have tradition-
ally been considered self-compatible (Mehlenbacher et al., 1991).
However, self-incompatible cultivars have been described by
several researchers. Schultz (1948) pointed out that ‘Riland’ and
‘Perfection’, two cultivars traditionally grown in the United States,
are self-incompatible. Later, many papers confirmed that, al-
though apricot populations in Europe and North America were
mainly self-compatible, self-incompatibility was more frequent
than had been thought. Lamb and Stiles (1983) found five self-
incompatible cultivars in a study of 19 cultivars suitable for
cultivation in New York, and Nyútjó et al. (1985) found one case
of self-incompatibility among 23 tested cultivars. Most recently,
several self-incompatible cultivars were found among those tradi-
tionally grown in Spain (Burgos et al., 1993; Egea et al., 1991).
Self-incompatibility in Prunus appears to be controlled by a
monogenic system with a multiallelic series (De Nettancourt,
1977). Data from almond (Dicenta and García, 1993; Socias i
Company, 1991) and sweet cherry (Crane and Brown, 1937) are
consistent with this hypothesis. Also, preliminary results indicates
the same system operates in apricot (Burgos, 1995). The locus
controls self-incompatibility and intraspecific cross-incompatibil-
ity (Kester et al., 1994; Socias i Company, 1991).
The increasingly narrow genetic base of breeding programs, a
consequence of using a small number of cultivars as parents, has
caused the frequent appearance of cross-incompatibility between
cultivars of some species. Early work by Tufts and Philp (1922)
established two incompatibility groups in almond. Much later, the
work of Kester and collaborators made it possible to establish new
incompatibility groups and assign additional cultivars to the groups
described earlier (Kester, 1963; Kester and Asay, 1975; Kester et
al., 1994). This was mainly due to the use of ‘Nonpareil’ and
‘Texas’ (‘Mission’) as common ancestors of many of these culti-
vars.
In apricot, only one case of cross-incompatibility has been
reported between Spanish cultivars: ‘Moniquí Fino’ and ‘Moniquí
Borde’ (Egea et al., 1991). The names of these cultivars indicate a

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J. AMER. SOC. HORT. SCI. 121(6):1002–1005. 1996.
For controlled crosses in the orchard, several branches with a
total number of about 200 flowers in the balloon stage were chosen
on each parent tree. Open flowers and immature buds were
removed, and flowers at the balloon stage were emasculated to
prevent self- and cross-pollination. Controlled pollinations were
carried out using a small brush and the corresponding pollen from
each male parent. After 8 weeks, fruit were counted and fruit set
percentages were determined.
For laboratory testing, branches of the cultivars to be pollinated
with about 30 flowers each in the balloon stage were cut and placed
in plastic bags containing water. The bags were transported to the
laboratory in an insulated ice chest. Once in the laboratory, the
basal ends were placed in beakers containing a 5% sucrose solution
in a chamber where the temperature was maintained at 20 °C. The
flowers were emasculated and 24 h later they were self- or cross-
pollinated. Also, branches with a similar number of flowers in
balloon stage were marked on the tree and the flowers were
emasculated. After 24 h, the flowers were pollinated, for compari-
son with the laboratory controlled pollinations. Seventy-two hours
after controlled pollination of the flowers transported to the labo-
ratory and 192 h after controlled pollination of the flowers on the
tree, 15 pistils per combination were harvested and immersed in
FAA (90% alcohol at 70%, 5% formaldehyde at 40%, 5% glacial
acetic acid) in small glass bottles. The pistils were stored at 4 to 5
°C in this solution. According to Williams (1970), 192 h is a
sufficient time for the pollen tubes to reach the ovary in compatible
Fig. 1. Parentage of the apricot cultivars used in this study (Layne, Ledbetter, and
Howell, personal communication). Unless indicated otherwise, the female parent
is at the top and the male parent is at the bottom. OP = open-pollination.
Table 1. Apricot pollen tube growth 72 h after self- and cross-pollinations under laboratory controlled conditions.z
Ovaries reached Avg no. Percent of style length
Pollinated Pollinizing by pollen tubes of pollen tubes penetrated by the
cultivar cultivar (%) in the ovary longest pollen tube
Lambertin-1 Lambertin-1 0 ± 0.0 0 ± 0.0 76.1 ± 8.0
Goldrich 0 ± 0.0 0 ± 0.0 86.3 ± 6.1
Hargrand 0 ± 0.0 0 ± 0.0 69.7 ± 16.7
Harcot 100 ± 0.0 6.2 ± 3.2 100 ± 0.0
Goldrich Goldrich 0 ± 0.0 0 ± 0.0 72.5 ± 11.9
Lambertin–1 0 ± 0.0 0 ± 0.0 54.3 ± 12.3
Hargrand 0 ± 0.0 0 ± 0.0 82.9 ± 9.0
Harcot 100 ± 0.0 11.4 ± 1.5 100 ± 0.0
Hargrand Hargrand 0 ± 0.0 0 ± 0.0 69.2 ± 9.8
Lambertin-1 0 ± 0.0 0 ± 0.0 ---
Goldrich 0 ± 0.0 0 ± 0.0 74.8 ± 9.9
Harcot 100 ± 0.0 3.9 ± 2.0 100 ± 0.0
Harcot Harcot 0 ± 0.0 0 ± 0.0 53.9 ± 12.7
Lambertin-1 100 ± 0.0 3.7 ± 2.0 100 ± 0.0
Goldrich 100 ± 0.0 7 ± 2.9 100 ± 0.0
Hargrand 100 ± 0.0 4.2 ± 1.9 100 ± 0.0
zValues are means from 10 samples ± SD.

1004 J. AMER. SOC. HORT. SCI. 121(6):1002–1005. 1996.
combinations at the daily average temperatures of about 15 °C
occurring during our orchard pollinations. To remove the FAA, the
pistils were rinsed in distilled water for three times 1 h each, and
then placed in an autoclave for 30 min at 1 atmosphere in a 5%
sodium sulfite solution to soften the tissues and facilitate their
staining with 0.1% aniline blue in 0.1 N potassium phosphate. The
pistils were stained for 24 h. The epidermis was then removed, and
the pistils squashed for observation (Linskens and Esser, 1957).
An Olympus BH2 microscope (Olympus, Tokyo) was used
with a BH2-RFL-T2 ultraviolet light source, using an Osram HBO
100 W/2 high-pressure mercury lamp (Osram GmbH, Berlin–
Munich).
Results
Ten of the sixteen pollinations performed in the laboratory were
incompatible (Table 1). In incompatible crosses, pollen tube
growth was arrested about three-fourths of the way down the style.
The percentages of fruit set following self-pollination confirm that
‘Goldrich’, ‘Hargrand’, ‘Harcot’, and ‘Lambertin-1’ are self-
incompatible (Table 2) and that the first three are cross-incompat-
ible in all possible combinations. ‘Harcot’ was compatible with the
other three cultivars. No pollen tube reached the ovary in the six
different combinations between ‘Lambertin-1’, ‘Hargrand’, and
‘Goldrich’ (Table 1). Orchard cross-pollination confirmed the
laboratory results and no fruit set was obtained in those combina-
tions tested except for very low fruit set (1.9%) in the combination
‘Lambertin-1’ x ‘Hargrand’ (Table 2). Results of the incompatible
combinations (self- and cross-) strongly contrast, in laboratory and
field pollinations, with those from the compatible ones, where
100% of the ovaries had a high average number of pollen tubes
(Table 1) and large percentages of fruit set were obtained (Table 2).
Some differences were found in the length of the style that
pollen tubes were able to reach in the incompatible combinations.
The percentages of style reached by the longest pollen tube ranged
from 48.8% in the cross ‘Goldrich’ x ‘Lambertin-1’ to 86.3% in the
reciprocal cross. These differences are probably genotype-combi-
nation dependent. Table 3 shows self-compatible and self-incom-
patible cultivars that were self-pollinated in the orchard to assess
results from laboratory controlled-pollinations. Very clear differ-
ences in the growth of pollen tubes were seen. In self-compatible
cultivars, pollen tubes grew the entire length of the style and
entered the ovary, while in self-incompatible cultivars tube growth
stopped in different parts of the style, depending on the cultivar.
Pollen tube growth in flowers pollinated in the field and
collected and fixed 192 h later was the same as on flowers
pollinated in the laboratory (Table 3).
Discussion
The three methods used to determine compatibility correlate
well and may be used interchangeably (Burgos et al., 1993),
although laboratory pollinations provide more controlled condi-
tions and help avoid uncontrolled cross-pollinations and unfavor-
able weather conditions.
The self-incompatibility of ‘Goldrich’ was previously described
by Lamb and Stiles (1983). No information on ‘Harcot’s’ self-
incompatibility was given when the cultivar was released (Layne,
1978) or by Lamb and Stiles (1983), who nevertheless included
‘Harcot’ in a study of apricot cultivars suitable for cultivation in
New York, where they described this trait for other apricot culti-
vars. In contrast with our results, ‘Hargrand’ was described as self-
compatible when released (Layne, 1981) as was ‘Lambertin-1’
(Audergon et al., 1988).
The first reported case of cross-incompatibility in apricot was
described between ‘Moniquí Fino’ and ‘Moniquí Borde’ (Egea et
al., 1991). However, their names and morphological similarity
indicate that they may be slightly different clones of the same
cultivar.
The self- and cross-incompatibility results cannot be explained
by a lack of pollen germination, since pollen germination took
place in all crosses and pollen tube growth was arrested in the style.
Also, all cultivars when crossed with ‘Harcot’ as female parent
showed good pollen germination on the stigma (data not shown),
and the number of pollen tubes reaching the ovary was high, as was
the percentage of fruit set.
The low percentage of fruit set obtained after self-pollination of
‘Lambertin-1’ (1.7%) and the combination ‘Lambertin-1’ x
‘Hargrand’ (1.9%) may have been caused by bees visiting the
flowers, despite the flowers’ being emasculated (Williams et al.,
1984). It could also be a case of pseudocompatibility, which has
been reported as a nongenetic condition that is environmentally
influenced by external (i.e., temperature) or internal (i.e., flower
age) conditions (Van Gastel, 1976). Examples of pseudo-compat-
ibility include young flower buds that have not yet developed the
incompatibility barriers or older flowers in which these are no
longer effective (Van Gastel, 1976; Williams and Maier, 1977).
Similar results have been obtained artificially by keeping the
flowers at specific temperatures during pollen tube growth (Wil-
liams and Maier, 1977) or by heating the pistil at specific periods
(Hiratsuka et al., 1989).
A model for the inheritance of incompatibility in Prunus has
Table 2. Fruit set percentages obtained from controlled self- and cross-
pollinations of apricot cultivars.
Pollinated cultivar Pollinizing cultivar Fruit set (%)
Lambertin-1 Lambertin-1 1.7
Harcot 73.7
Hargrand 1.9
Goldrich 0
Hargrand Hargrand 0
Harcot 19
Lambertin-1 0
Goldrich 0
Harcot Harcot 0
Goldrich 62.5
Lambertin-1 65.9
Hargrand 36.7
Goldrich Goldrich 0
Table 3. Percentage of pistil reached by the longest pollen tube 192 h after
self-pollination of self-compatible and self-incompatible apricot cul-
tivars in field conditions.z
Self-pollinated Percentage of style penetrated
cultivar by the longest pollen tube
Self-incompatible cultivar
Lambertin-1 69.2 ± 10.3
Harcot 57.7 ± 17.5
Moniquí 81.8 ± 7.6
Self-compatible cultivar
Bebeco 100 ± 0.0
Canino 100 ± 0.0
zValues are means from 10 samples ± SD.

1005
J. AMER. SOC. HORT. SCI. 121(6):1002–1005. 1996.
been proposed as a homomorphic, monofactorial, and gameto-
phytic system (De Nettancourt, 1977). Preliminary results in
apricot (Burgos, 1995) seem to confirm this hypothesis, where a
locus with a multiallelic series would control the trait. Hence,
cross-incompatible cultivars would have the same genotype for the
trait. We propose designating those two alleles as S1 and S2.
The three cross-incompatible cultivars have ‘Perfection’ among
their ancestors. This is an old cultivar used extensively in North
American breeding programs because of its large size, attractive
orange color, and firm flesh. ‘Perfection’ is a parent or grandparent
of some recently introduced cultivars such as ‘Rival’ (Brooks and
Olmo, 1971), ‘Tracy’ (Brooks and Olmo, 1972b), ‘Skaha’,
‘Sundrop’, and ‘Westley’ (Brooks and Olmo, 1975) or ‘Castlebrite’
(Brooks and Olmo, 1978). It would be interesting to identify the
alleles in ‘Perfection’, but unfortunately this cultivar was not in our
apricot collection. ‘Goldrich’ (S1S2, as proposed) and ‘Perfection’
(S1S3) have an allele in common since ‘Goldrich’ was obtained
from the cross ‘Sun Glo’ x ‘Perfection’. If ‘Goldrich’ and ‘Perfec-
tion’ are cross-compatible, seedlings from this cross would be of
two different genotypes (i.e., two incompatibility groups) since S1
alleles would be arrested. One of these groups would be the same
as the male parent (S1S2 or S1S3 if ‘Goldrich’ or ‘Perfection’
respectively, are the male parent), while the other one would be
new (S2S3). These cultivars may also belong to the same incompat-
ibility group and be cross-incompatible if ‘Perfection’ and ‘Sun
Glo’ share one allele. Crosses of ‘Goldrich’ with unrelated culti-
vars would give genotypes representing four incompatibility groups.
It is only by making controlled crosses and identifying incompat-
ibility groups in these progenies that alleles can be assigned to
groups.
The narrow genetic base resulting from using a limited number
of parents has resulted in a small number of incompatibility groups
in almond or cherry (Kester et al., 1994). Incompatibility is
difficult to detect and is also undesirable, since incompatible
clones require pollenizers and render the yield dependent on
abundant pollen transfer among the trees (Frankel and Galun,
1977). The appearance of cross-incompatibility in this group of
cultivars indicates that greater attention should be given to this
character in breeding programs. The laboratory pollination proce-
dure described here would allow quick and easy identification of
self-incompatible genotypes.
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